A dependency for a specific platform can be achieved with e.g.:
[target.'cfg(windows)'.dependencies]
foo = "0.1"
A feature-enabled dependency can be achieved with e.g.:
[dependencies.foo]
version = "0.1"
optional = true
[features]
feature = ["dep:foo"]
How can you combine both? That is, a dependency that is enabled by default on one platform, but can be enabled on all platforms via a feature?